There exists a biometric authentication technology that uses personal biometric information such as hand veins, a fingerprint, an iris, a face or the like, as an authentication method of a person who is entering or leaving a room, a facility or the like, without human intervention. Authentication using biometric information has an advantage over authentication using a magnetic card or a personal identification number (PIN) in that it does not need to care about loss and fraudulent use.
For example, basic vein authentication starts with illuminating near infrared light onto a part (for example, a palm) of a human body where an image of veins can be easily captured, captures strength distribution of reflected or transmitted light, and extracts the image of veins (blood vessels) by characteristic extraction based on the strength distribution. Also, vein authentication verifies the extracted image of veins (called “data to be verified”, hereafter) with the image of veins of the person registered in advance (called “registered data”, hereafter) to determine whether they are coincident with each other, and outputs an authentication result.
Here, swell (concavity and convexity) of muscles at the base of fingers change depending on the form of the fingers when the image is captured, such as open fingers, closed fingers, or bent fingers. In the following, the swell part is called a “pliable part”. At a pliable part, it is difficult to extract a satisfactory image of veins because near infrared light used for capturing an image is illuminated on the surface of skin obliquely and the light is scattered due to thickness inside of skin and muscle, which degrades authentication precision.
Conventionally, there exist methods to cope with the above problem such as a method of providing a guiding mechanism to instruct a hand posture for opening a palm, a method of detecting a degree of openness of a palm to guide a user to take a posture suitable for authentication, and a method of capturing images of a palm repeatedly so that the most suitable image for authentication can be selected (see, for example, Patent Documents 1 to 4).